Practically all electronic equipment being designed today must meet ever-more-stringent design. requirements, such as higher efficiency, smaller size, and immunity to power line variations. These constraints have been brought about by rising energy costs and the greater frequency of occurrence of a "brown-out" condition, where the voltage is deliberately reduced by some amount, or a "black-out" condition, which is an outright failure of the line voltage. Therefore, in order to protect equipment from the harmful consequences of such occurrences, it is necessary to provide a signal warning about the power fail condition. This is especially true for equipment such as electronic computing equipment which contains volatile memory and requires particularly fast warning concerning such occurrences, so that steps can be taken to maintain the operation of the equipment in an uninterrupted fashion or at least take measures to protect the information stored in the volatile memory.
One known detection method widely used consists of detecting when the peak of an AC line cycle has dropped below a preselected level and then generating a power fail signal when too many of such low voltage cycles occur within an arbitrary time period. An exemplary solution, illustrated in FIG. 1a, involves a brownout detector 10 (full wave rectifier 12, comparator 13 and missing pulse detector 15/16) to sense AC power failures. Such a detector is an improvement over a simple black-out detector which merely has a comparator for determining when the AC line voltage has dropped below a preselected level, and is well-known in the art. While either arrangement is suitable for some equipment designs, each assumes that the power supply contains a sufficiently large filter capacitor and a linear transformer having minimal energy-storage characteristics for interfacing with the AC line voltage.
The first disadvantage is that the power supplies themselves do not achieve efficiency levels and physical size requirements demanded by the design constraints of today's equipment.
One solution would be to increase the capacitance and hold time of the series pass regulator 208. The result would be a series pass regulator 208 that is operated farther from saturation and, therefore, less efficiently, at greater cost and with greater heat dissipation.
A second disadvantage of either of such arrangements especially when using a ferroresonant transformer is that the detection methods prove to be too slow to be effective during an AC line fail condition because the stored energy in a ferroresonant transformer 21 is dumped back into the AC power lines when the power is interrupted. Thus, ferroresonant transformers 21 present a sinusoid with an exponentially decaying envelope that continues to fool the missing pulse detectors 15/16 for up to 50 ms after an actual power failure.
Ferroresonant power supply transformers 21 are highly efficient because they, inherently, provide a constant output voltage. However, absent any active feedback, ferroresonant power supplies provide poor load regulation.
At the sacrifice of some power supply efficiency, effective load regulation can be achieved by following the ferroresonant transformer with a linear series pass regulator 208. Not much efficiency is sacrificed, however, when the series pass regulator 208 is operated very near saturation. However, in the nearly saturated series pass regulator, its internal filter capacitor offers very little hold time in the event of a power failure.
In installations having volatile memories, the short hold time of the filter capacitor in the nearly saturated series pass regulator 208 does not provide much time (worst case, 14 milliseconds) to respond with battery backup before the output falls out of regulation.
The need for this invention arose from problems of fast power-fail detection with ferroresonant power supplies and battery backup for volatile memories.
What is required, then, is a faster AC power fail detector.
The instant invention solves the problem by ANDing 27 a sag detector 26 with the brownout detector 25 before the missing pulse detector 28/29.
This invention represents a significant advance over the prior art and over this technical field by providing a fast power-fail detector for ferroresonant power supplies including, in combination, a brownout detector and sag detector.